1. Field of the Invention
The present invention relates to a filling-level measuring device. More specifically, the present invention relates to a filling-level measuring device having a housing and a measurement cell arranged therein, wherein the cell further comprises an essentially cylindrical measurement cell body with a membrane arranged crosswise relative to a longitudinal axis of the cell body. The measurement cell is mounted in the housing by means of a circumferential sealing element, which is arranged between the cell body and the housing.
2. Description of the Related Art
The related art involves filling level measuring devices. Such devices are known from the prior art and comprise a substantially cylindrical housing, in which a front-side measurement cell is arranged in the direction of a process environment. The measurement cell comprises a substantially cylindrical measurement cell body, which ordinarily comprises a front-side membrane arranged crosswise relative to a longitudinal axis of the measurement cell body. The measurement cell is mounted in the housing of the filling-level measuring device by means of a circumferential sealing element, which is arranged between the measurement cell body and the housing.
To be able to achieve sufficient seal tightness of the filling-level measuring device it is necessary that the sealing element, which, by way of example, can be provided as an O-ring, is compressed to develop a sealing effect. By way of the compression of the sealing element, forces are fed into the measurement cell in radial direction, which vary depending on the ambient temperature, which has a significant influence on an expansion of the sealing element. By means of such variation of the fed-in forces, an output signal of the measurement cell is distorted.
To date, it has been attempted to keep a change of the measurement signal—caused by strain as well as fluctuations in temperature—small, by placing the sealing element as far as possible behind the glass seam, the joining location between the membrane, and the measurement cell body. For a measurement cell with an extension of about 10 mm, the sealing element was thus arranged staggered to the back with a distance of 5 mm to the joining location. In addition, it was also attempted to feed thermally conditioned volume changes of the sealing element through a lower biasing voltage of the same to a lesser degree into the measurement cell body.
A lower biasing compression of the sealing element admittedly causes a lower temperature dependant zero point offset of the measurement signal; but, what is not appreciated by the prior art is that the lower biasing compression has simultaneous effects on the long term seal tightness of the measurement cell. Furthermore, with increasing age of the sealing element, its elasticity and with this a bias compression between the housing and the measurement cell body, decreases, so that a further offset of the measurement cell's zero point can be expected. A measurement exactness of the known filling-level measurement devices is thus limited by the aging of the sealing elements, as well.
Accordingly, there is a need for an improved filling-level measurement device which corrects the above referenced disadvantages and provides a thermally stable assembly with sufficient long term stability.